1. Field of the Invention
The present invention relates to a catalyst for purifying an exhaust gas. The present invention particularly relates to a catalyst for purifying an exhaust gas containing a complex oxide which does not deteriorate even in a lean atmosphere containing sulfur dioxide SO.sub.2 at a temperature as high as at least 1,000.degree. C.
2. Description of the Prior Art
In catalysts for purifying an exhaust gas of automobiles, etc., noble metals such as Pt, Pd and Rh are used singly or in combinations as the catalyst component. The catalyst component is usually supported on a catalyst support. For example, the present applicants have proposed in Japanese Unexamined Patent Publication (Kokai) No. 62-277150 that the use of a perovskite type complex oxide of Pt, and a lanthanoid element or alkaline earth metal element, or an analogous complex oxide, as a catalyst for purifying an exhaust gas of internal combustion engines prevents thermal deterioration and alloying of Pt, and improves the durability and the purification performance.
Although the durability, etc., can be significantly improved compared with the prior catalysts, the perovskite type complex oxide starts to decompose when the exhaust gas temperature exceeds 900.degree. C. The exhaust gas temperature in automobiles has been greatly increased, due to various kinds of exhaust gas regulations enacted in recent years, and the development of a catalyst which can sufficiently purify an exhaust gas even at temperatures exceeding 1,000.degree. C. is desired.
Although the initial object has been accomplished by a catalyst which is a highly heat-resistant complex oxide containing Pt and which has been proposed as the desired catalyst by the present applicants in Japanese Patent Application No. 8-184465, it has been found that the proposed catalyst cannot produce sufficient effects when the exhaust gas contains SO.sub.2. Accordingly, a catalyst exhibiting sufficient durability even when the exhaust gas contains SO.sub.2 is desired.